1. Field of the Invention
The present invention relates to a soft start circuit and a power supply, more specifically, to a soft start circuit for controlling an output of a pulse width modulation controller, and a power supply including the soft start circuit.
2. Description of the Related Art
A power supply is provided to fiat panel displays including plasma displays and liquid crystal displays (LCD), and various electrical and electronic devices that require a power source of a predetermined level. A power supply using a pulse width modulation (PWM) method among the power supplies includes a power converter and a pulse width modulation controller.
The power converter generates an output voltage based on an input voltage, and there are different kinds of power converters existing such as a half bridge converter operating with a high switching frequency and a low switching loss and operating asymmetrically, a power converter having an output inductor and such power converter including a positive clamp forward converter and a phase shifting full bridge converter, and a power converter without output inductor.
A pulse width modulation controller monitors the output of the power converter, and controls the power converter so that the output remains at a predetermined level. That is, the pulse width modulation controller controls the operation and the output of the power converter from an initial start-up operation state to a normal operation state, then to an operation stop state.
Generally, the power converter receives an operational signal having a 1:1 duty ratio according to a control operation of the pulse width modulation controller in the normal operation state. That is, the power converter performs a normal operation according to the operational signal having 1:1 duty ratio.
When the power converter operates according to the duty ratio of the normal operation state in the initial start-up state, an overload, however, may occur at an output terminal of the power converter. Therefore, a soft start method has been suggested. In the soft start method, the power converter operates with a driving signal with a low duty ratio, and the duty ratio is gradually increased and becomes equal to the duty ratio of the normal operation state.
When the duty ratio of the operational voltage signal of the start-up state is set to be lower than that of the normal operation state, a current of a primary side of a transformer increases more quickly, and has a higher maximum peak value than in the normal operation state.
In addition, according to characteristics of the power converter, a current flowing through the second side of the transformer is induced by the current flowing through the primary side thereof, and therefore waveforms of currents at the secondary side of the transformer is proportional to the waveforms of current at the primary side of the transformer. Accordingly, a current having a higher peak value may be generated at the secondary side of the transformer in the start-up state compared to the normal operation state, and therefore electrical elements may be deteriorated by the current of the high peak value of the start-up state in the power converter using the soft start method.
Particularly, compared to the power converter having the output inductor, the elements may be more easily deteriorated in the power converter without the output inductor for delaying a flow of the current induced to the secondary side of the transformer.
The above information disclosed in this background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.